Signal selector



17, 1961 L. J. M KEssoN 2,968,718

SIGNAL SELECTOR Filed Jan. 28, 1957 3 Sheets-Sheet. 1

INPUT A SELECTIVE DIODE DETECTOR GATE :0

48 RECEIVER (S) 14' 22 [/6 LIMITER SELECTIVE DIODE E 28 INPUT B DETECTORGATE A 32 E 80 O I! [Ll IL E 40 D O o 20 O ,2 3 g .5 -6-7.8.9| 2 3 4 5 6a w SIGNAL TO NOISEtRMS) DB RATIO INVENTOR. V LEWIS JAMES IIG'KESSON ATTOR NEY5 Jan. 17, 1961 L. J. MCKESSON SIGNAL SELECTOR 3 Sheets-Sheet 2Filed Jan. 28, 1957 MQE F 333.55 u zbmdw Nw M E V m SQQJ. h: .m

m zutzj sT Wm M. v 5.223

LEWIS JAMES HcKESSOM AT'TQRNEYS Mu Pani- I IfPDLE Jan. 17, 1961 J.McKEssoN 2,968,718

SIGNAL SELECTOR Filed Jan. 28, 1957 3 Sheets-Sheet 3 AT TORNEXS UnitedStates Patent SIGNAL SELECTOR Lewis James McKesson, Garden City, N.Y.,assignor to Crosby Laboratories, Inc., Hicksville, Long Island, N.Y., acorporation of New York Filed Jan. 28, 1957, Ser. No. 636,617

18 Claims. (Cl. 250-20) This invention relates to signalling, especiallyby voice, and more particularly to improving the degree of intelligenceconveyed thereby.

'One object of this invention is to improve the percentage ofintelligence of a voice radio signal which has been degraded byextraneous causes. Such causes include static interference; intentionalinterference or jamming; non-intentional interference; fading; andmultipath distortion.

A more specific object is to provide a method of select ing the betterof two or more voice signals, and delivering this better signal toanoutput circuit where its intelligence is used. One source of thesignals can be two or more receivers operating in conjunction with spaceor frequency diversity. Another source can be the sidebands of anamplitude or phase modulated signal in which the sidebands have beenseparated by suitable filters. Other sources may be used for multiplesignals from which it is desired to select the one with the greaterpercentage of usable intelligence.

The type of intelligence transmitted is voice or audio signals. Thespecific embodiment shown was constructed for use at audio frequencieshaving a spectrum or range of approximately 30 to 10,000 cycles persecond.

Study of amplitude modulated signals shows that because of the differentfrequency spectrums occupied by the two sidebands, deleterious effectssuch as jamming, interference, noise and fading do not always affectboth sidebands equally. An object of. the invention is to provideequipment to select the better sideband, whereby an overall improvementis obtained.

In order for an equipment to decide which is the better sideband, itmust be able to recognize the difference between the two. Thisdifference may be on the basis of strength, or it may be on the basis ofsignal to noise ratio. Ideally, such a device used for a speech signalwould always select that one which has the more intelligence, i.e.,which is the more understandable; however, an electronic brain tosatisfy this requirement is beyond practical limits at present.

In accordance with a feature and object of the present invention, theselection is controlled by gates, which are controlled by a fiipfiopswitch, which in turn are controlled by selective detectors. These areeach essentially a filter followed by a rectifier, and the filterprovides a flat top or band pass selection of the maximum energy portionof the complete audio spectrum. Thus although the audio spectrum may befrom 30 to 10,000 cycles per second, the maximum energy usually isconcentrated in a limited range of say 300 to 600 cycles per second. Bybasing the selection on this maximum energy component, the effect ofrandom noise all over the spectrum is minimized, and maximumintelligence is obtained at the output.

A specific object is to build an electronic device which will operate oneither amplitude ratios, or on signal to noise ratios. Such a device canbe used to deliver any one of four types of output as follows:

- and vice versa.

2,968,713 Patented Jan. 17, 196i (1) The stronger of two signals, or

(2) The weaker of two signals, or

(3) The signal with less noise, or

(4) The signal with rnore noise.

For this purpose I provide limiters ahead of the selective detectors,with means to disable the same at will, and I also provide a phasereversing means between the fiip-fiop switch and the gates controlledthereby.

While this device perhaps is most useful in selecting one sideband of adouble sideband amplitude modulated or phase modulated signal, it alsomay be used to select between the outputs of two space diversity orfrequency diversity receivers, or the better output as between anamplitude modulation receiver and a phase modulation receiver. Theselection on the latter basis (amplitude modulation versus phasemodulation) gives considerable improvement on high frequency signals.Observations indicate that because of selective fading of the twosidebands, phase rotations are such that when an amplitude modulationdetector gives maximum undistorted output, a phase modulation detectorwill usually give a minimum, In other words, amplitude modulation andphase modulation at the input of the receiver tend to be out of phase,so that one is almost always better then the other.

To accomplish the foregoing general objects, and other more specificobjects which will hereinafter appear, my invention resides in thesignal selector apparatus and the elements thereof, and their relationone to another, as are hereinafter more particularly described in thefollowing specification. The specification is accompanied by drawings,in which Fig. 1 is a block diagram showing one form of the invention;

Fig.2 is a graph explanatory of the invention;

Fig. 3 is a detailed wiring diagram corresponding to the blocks 12, 14,20 and 22 in Fig. l; and

Fig. 4 is a detailed wiring diagram corresponding to the blocks 24, 26and 28 in Fig. 1.

Referring to Fig. 1, the two signal inputs may be supplied at terminals30 and 32. The selected signal is delivered at the output terminal 34.The input signals ordinarily are obtained from a diversity receiver (orreceivers) indicated by block 42. If the reception is by spacediversity, there will be separate and widely spaced antennas 44 and 46,leading to separate receivers, symbolized in block 42 by the dottedpartition line 48. If the reception is single side band diversity, it iscustomary to think of the receiver as a single receiver, but it has twoseparate outputs, one for one side band, and one for the other sideband, with only one antenna system required. Generally similar remarkapplies to reception by frequency diversity, or by amplitudemodulation-phase modulation (AM/ PM) diversity. The reception also maybe by amplitude modulationfrequency modulation (AM/FM) diversity. In anycase, the block 42 represents any suitable receiver apparatus to deliverthe desired two separate outputs, one or the other of which is to beselected by the present apparatus.

The apparatus comprises optionally usable limiters 12 and 14, followedby selective detectors 20 and 22,*wh1'ch control a means 24, 26, 28 toselect the desired signal in response to a control potential obtainedfrom the selective detectors. As here illustrated I employ a flip-flopswitch 24, which in turn controls diode gates 26 and 28. One gate isclosed when the other is open, and vice versa, thus feeding one only ofthe two inputs at 30 and 32 to the output terminal 34.

The selective detectors are essentially narrow bandpass filters followedby rectifiers, preferably double diodes providing a DC. output. Thefilters provide a substantially fiat top resonance curve for selecting adesired narrow part of the broad audio frequency-spectrum. I believe arange of 440 to 600 cycles per second is the maxi mum energy part of theusual voice wave.

I shall first describe the selection of either the stronger or weaker oftwo signals. Referring to the block diagram, Fig. 1, this isaccomplished by shorting out the two input limiters, 12 and 14, which isdone by closing the switches 16 and 18, and adjusting the input voltagesfor proper operation. For voice operation, the selective detectors 20and 22 are tuned preferably to a band pass range of 440/600 cycles persecond, as above indicated. The output from the rectifiers in theselective detectors 20 and 22 is used to flip the flip-flop circuit 24,which in turn operates the two gates 26 and 28.

To operate the switching apparatus in a manner based on signal to noiseratios, the limiters 12 and 14 are required. The switches 16, 18therefore are opened, The limiters will then be applying their limitingprocess to the combination of the audio signal and the noise. Under suchcircumstances, I have found that, when a portion onlyof the bandwidth isselected at the output of the limiter, the detected amplitude of thisselected portion decreases as the signal-to-noise ratio decreases.Accordingly, the output of the selective detectors becomes proportionalto signal-to-noise ratio. As a result, the potential obtained from theoutput of the selective detectors may be used to control signalselection in accordance with signal-to-noise ratio.

- The curve 38' in Fig. 2 shows a typical measurement of the reductionin DC. output as the signal to noise ratio is reduced. It will be notedthat the output does not go down to zero in the absence of signal. Thisis so because of the finite bandwidth of the selective detectors. As thebandwidth" of the selective detectors is increased, the slope of theoutput curve 38 changes. The output can be made zero by applying athreshold bias on the rectifiers in the selective detector, so that theeifect of infinitely sharp (Q) tuned circuits is simulated.

Normally, this threshold bias is so adjusted that random noise producesno output. Then, theoretically, any signal will produce an output.However, in practice, signals less than 20 db below the noise level arenot sufficient to give usable differentials for the switching operation.The actual setting of the threshold and the diiferential voltagenecessary to shift from one channel to the other is dependent on thetype of signal and noise. It seems that one setting is not suitable forall operating conditions.

The circuit shown in block form in Fig. 1 is shown in greater detail inFigs. 3 and 4. The upper input #1 (Fig. 3) goes to gate 26 (Fig. 4) bymeans of conductors 40. The lower input #2 (Fig. 3) goes to gate 28(Fig. 4) by means of conductors 42. The gates control which input is fedto the output (Fig. 4). The two conductors 40 and the two conductors 42are each preferably shielded by means of grounded shielding, throughouttheir length.

The wiring in blocks 12 and 14 is substantially identical, and theseunits contain the limiters. For strongest/ weakest signal operation theyare not in use, and are disabled by switches S3 and S4, which correspondin function to the switches 16 and 18 in Fig. 1. Another switch S2 atthe upper limiter is used to reverse the phase of one signal withrespect to the other. It is closed in such direction that the outputdoes not go through a sudden phase reversal when shifting from onechannel to the other, as is discussed later under Operation. Thelimiting action of the limiters may be controlled, as by varying thepotentials applied to the tubes to obtain saturation at a desired level.

In the particular circuitry here shown the tubes V1 and V2 are type6AU6; the resistors R1 and R2 are 470K ohms; the resistors R3 and R4 are220 ohms; the resistors R5, R6, R7 and R8 are all 100K ohms; and theresistor R9 is l megohm. The capacitors C1 and C2 are 0.05 mf.; thecapacitors C3 and C4 are 0.002 mf. The plate spasms 4 potential is 200volts at 6 milliamperes. The transformers are United Transformer CompanyUT C/LS-26.

The component values in lower block 14 for channel #2 are the same asthose in upper block 12 for channel #1. It will be understood that thequantitative values given here, and later on for the other blocks, aresolely by way of example, and not intended to be in limitation of theinvention.

The next block contains the selective detectors 20 and 22, with athreshold control R17. The matter of adjustment of the threshold biashas been discussed above. A balance control R16 is used to set theoutput voltages'of each channel, and to compensate for unsymmetricalcircuitry or balance of the inputs. The band pass filters comprise tworesonant circuits preferably tuned to the ends of the desired band. Inthe present case inductor L1 and capacitor C5 are tuned to 600 cycles,while inductor L2 and capacitor C6 are tuned to 440 cycles. This resultsin a flattened top for a range of 440 to 600 cycles.

The selective detectors use full wave rectifier tubes V3. In theparticular case here shown, the tube V3 is a type 6AL5; the resistorsR14 and R15 are K; the potentiometer R16 is 50K; the potentiometer R17has a resistance of 25K ohms; the resistor R22 is 1 megohm; the resistorR23 is 56K; the capacitors C5 and C6 are each 0.05 mf.; the capacitor C7is 0.03 mf.; the capacitor C8 is. 0.01 mf.; and the inductors L1 and L2are variable tuners made by United Transformer Company as type V1C13.The potential supplied to potentiometer R17 is 200 volts at 8milliamperes at resistor R23.

The components in the lower channel #2 (corresponding to block 22 inFig. 1) have the same values as here given for the upper channel #1 ordetector 20.

Referring now to Fig. 4, this contains the flip-flop switch in block 24,and the diode gates in blocks 26, 28. Referring to block 24, theflip-flop tube V4 is a dual triode. The circuit network around the tubemay be conventional for this purpose. It applies apositive potential tothe plate of one diode gate, and a negative potential to the other. Thecombination is an inertialess electronic relay with instantaneouschangeover. Resistor R21 is the flip-flop cathode resistor, and itssetting determines the sensitivity of the flip-flop action, and thedifferential in input voltages necessary to switch. For signal/ noiseratios in the range of zero decibels, this switching can occur on lessthan one decibel difference.

In the particular flip-flop circuit here shown, the tube V4 is a type12AT7; the resistors R10 and R11 are 470K, the resistors R12 and R13 are47K ohms at 2 watts; the resistors R18 and R19 are 33K; and the variableresistor R21 is 5K. The plate potential is supplied at 200 volts.

The apparatus may include the two neon indicator lamps 50 and 52connected to the switching tube plates. With switch S5 in the normalposition, the lighted lamp indicates which channel is furnishing output.With switch S5 in the reverse position, the dark indicator lampindicates the channel which is in use. The lamps shown are neon lamps,with suitable series resistance.

Switch S5 reverses the action of the circuit, so that one may selecteither the stronger or weaker signal, when working on a signal strengthbasis, or one may select the signal with either the greater or lessernoise, when working on a signal to noise ratio basis.

The circuitry of the diode gates is shown in box 26, 28. Thepotentiometer R20 so adjusts the bias on the cathodes of the doublediode gates that the one passing the signal will not distort because ofcut-ofi during part of the cycle. Either gate tube V5 alone or gate tubeV6 alone is given positive plate potential by the flip-flop circuit, inorder to make one gate tube conductive, while the other gate tube ismade non-conductive by negative potential on its plates. 7

In the specific circuit shown, the tube V5 is a type 6AL5; the threetransformers are Kenyon type T26; the

potentiometer R20 is 25K ohms; the potential supplied to thispotentiometer is 200 volts; and the resistors R25 and R26 are 560 ohms.The tube V6 and its surrounding components for channel #2 are the sameas those for channel #1.

The power supply is shown in block 60 in Fig. 3. It may be conventional,using a transformer T4 with a low voltage secondary 62 to heat thecathodes of the tubes, and a high voltage secondary 64, rectifier V7 andfilter L3, C9, R24 to supply B potential for plate supply. This is at200 volts in the particular case shown, at 33 milliamperes. The filamentpotential is at 6.3 volts.

In the particular case here shown the transformer is a UTC (UnitedTransformer Company) number HP- 123; the tube V7 is a type 6X4; theinductor L3 is a UTC/LS-90; the capacitor C9 is 30 mf.; the resistor R24is 10K at 10 w.; and the lamp 66 is a pilot lamp or indicator.

It may be mentioned that the previously suggested audio frequency rangeof 440 to 600 cycles is not critical, and that a range of say 300 to 500cycles might be used, or 300 to 600 cycles.

Operation After setting up and applying power, the selective detectortuning should be checked, as by connecting an audio oscillator to oneinput, and a vacuum tube voltmeter from N on the chassis (Fig. 3) behindunit 20 selective detector) to ground. Adjust the audio oscillator to440 cycles per second and tune L2 for maximum voltmeter readings.Repeat, using 600 cycles per second from the audio oscillator, but thistime tuning L1.

Selective detector 22 is then tuned in the same manner, with theindicator on terminal R (Fig. 3).

The operation of each of the limiters 12 and 14 canbe checked byreducing the input from 0 dbm to -40 dbm, during which the selectivedetector outputs should remain essentially constant. The switches S3 andS4 are closed to the left in Fig. 3.

Next parallel the two inputs and apply 600 cycles per second. Set thebalance control R16 to center position. Set R17 to about mid position.Adjust R21 until the switcher or flip-flop circuit operates, asindicated by one or the other of the indicator lamps 50, 52 just goingout. With S5 in the normal position for signal-to-noise ratio operation,next adjust R20 until minimum distortion is present in the output, asindicated by an oscilloscope. The adjustment of R20 may spoil thefiip-fiop action of the tube V4, that is, R21 may require readjustment,which should be done.

After making the rough adjustments above, fine adjustments are madeuntil the fiip-fiop switching can be accomplished by very slightadjustments of R16 to either side of the mid-position.

Using a mixer amplifier, white noise, that is, of many variedfrequencies, should next be added to whichever channel is then switchedto the output, and the noise level raised until the flip-flop circuitoperates to connect the side without noise, to the output. This noiselevel should be about db below the signal. Transferring the noise to theother channel should reverse the action.

The proper setting of the phase switch S2 can be de termined by applyinga pure tone (the same tone) to each input and switching back and forthby moving R16 slightly. When the switch S2 is in the wrong position aclick will be heard or seen in an oscilloscope.- In the right positionthe click cannot be detected.

To operate on the basis of stronger/weaker signal, one should throwswitches S3, S4 and S5 to opposite position, and reduce R17 to zero.(Switches S3 and S4 are closed to the right in Fig. 3. Under theseconditions the, unit will select the weaker signal, i.e., the signalwhich has the least energy in the pass band of the selective detector.Some readjustments of R16, R21 and R will probablybe necessary foroptimum operation.

The dark indicator lamp will indicate the channel switched in.

In either mode of operation care should be taken to keep the inputvoltages low enough to prevent distortion which otherwise will occur inthe diode gates.

The apparatus may then be tested on a radio signal, such as the upperand lower sidebands fed through a sideband adapter, or two spacediversity receiver outputs, or an amplitude modulation and a phasemodulation output from a Crosby sideband adapter unit, say Type 51. Thebest mode of operation is dependent on the character of the signal andnoise. If the jammer is a discrete carrier staying on one sideband, orwobbling back and forth (even as fast as at audio rates), the switcherwill follow, and furnish the best signal. For random noise the operationis less positive.

The operation of the fiip-fiop and gate control may be explained withreference to Figs. 3 and 4, which may be placed end to end to form thecomplete diagram. The

upper selective detector V3 (corresponding to box 20 in Fig. 1)rectifies the output of the selective filter C5, L1, 06, L2, disposed inthe output of the limiter 12. The same applies to the lower half of tubeV3 (which corresponds to box 22 in Fig. 1). The output of the upper halfof V3 appears onthe Wire N, and the output of the lower half of V3appears on the wire R. These two outputs are fed to the grids of theflip-flop tube V4.

If there is a stronger potential on the wire N this causes the upperdiode of tube V4 to draw current, and to pass potential through theswitch S5, to either the tube V5 or the tube V6 depending on which waythe switch S5 has been closed. Tubes V5 and V6 are the diode gates.

' If the said potential goes to tube V5, it causes its diode to becomeconducting and to pass output from the upper pair of wires 40 to thefinal output terminals, this being done through the transformers and theresistors R25 and R26. On the other hand, if potential goes to the gatetube V6 then the opposite occurs, and output is passed from the lowerpair of wires 42 through the transformers and the resistors R27, R28 tothe final output terminals. The flip-flop tube V4 acts as an electronicswitch which is conductive on one side and completely cut off on theother side, and therefore only one output or the other reaches the finaloutput terminals.

For a signal within the pass band, this unit will select either thesignal with the least noise, or the weaker of the two signals. On asignal-to-noise ratio basis, it will operate and select on a noisedifierential of less than 3 decibels when the noise varies from 15 dbbelow the signal to 10 db above the signal. On amplitude ratios it willselect the weaker signal on a differential of less than 2 decibels fromabout 30 db to 0 dbm (dbm meaning db above one milliwatt across 600ohms).

When the selection is on the basis of signal strength it is usual toselect the stronger signal. However, mention has been made above therather unusual. feature that the apparatus can equally well select theweaker signal, and there are occasions when it may be advantageous to doso. For example, in case the interference is intentional jamming, itsometimes happens that the jamming is more effective against thestronger signal, and less eifective against the weaker signal, so that agreater degree of intelligence may be obtained when selecting the weakersignal instead of the stronger signal.

The distortion introduced by the present apparatus is less than 5% oneither mode of operation, for a maximum output of 0 dbm.

. The above performance is for normal line voltages and frequencies.Tube selection as between one or another of the same type is notrequired. However, one tube, V4.

(the 12AT7), is critical as to balance, and if the two triodes in tubeV4 do not age equally, a tube change will be necessary in its socket.

It is believed that the construction and operation of apparatusembodying my invention, as well as the ad vantages thereof, will beapparent from the foregoing detailed description. I have discovered thatby tuning a selective detector to the maximum energy part of the voicerange, I can obtain a selection action which is controlled by the voicewave itself.

It will be understood that while I have shown and described my inventionin a preferred form, changes may be made in the circuitry disclosed,without departing from the scope of the invention, as sought to bedefined in the following claims.

I claim:

1. Apparatus for selecting from two audio frequency signals, the signalhaving the greater or lesser signal-tonoise ratio, said apparatuscomprising a diversity receiver to provide the signals, limitersreceiving the signals from the diversity receiver, selective detectorsreceiving the signals delivered by the limiters, said selectivedetectors being tuned to a limited portion which is the maximum energyportion of the complete audio spectrum, and means connected to saidselective detectors for selecting the desired signal in response to acontrol potential obtained from the said selective detectors.

2. Apparatus for selecting from two voice signals, the signal having thegreater or lesser signal-to-noise ratio, said apparatus comprising adiversity receiver to provide the signals, limiters receiving thesignals from the diversity receiver, selective detectors receiving thesignals delivered by the limiters, said selective detectors being tunedto a maximum energy portion of the complete audio spectrum, within, say,300 to 600 cycles per second, means connected to said selectivedetectors for selecting the desired signal in response to a controlpotential obtained from the said selective detectors, and a phasereversing switch included in said means whereby the latter may selectthe signal with more noise or the signal with less noise.

3. Apparatus for selecting from two audio frequency signals, the signalhaving the greater or lesser signal-tonoise ratio, said apparatuscomprising a diversity receiver to provide the signals, diode gatesreceiving said signals and connected to output terminals, a flip-flopswitch connected to said gates to make either gate conductive and theother non-conductive, limiters receiving portions of the signals fromthe diversity receiver, selective detectors receiving the signalsdelivered by the limiters, said selective detectors being tuned to amaximum energy portion of the complete audio spectrum, and the outputsof the detectors being connected to the flip-flop switch to control thegates.

4. Apparatus for selecting from two audio frequency signals, the signalhaving the greater or lesser signal-tonoise ratio, said apparatuscomprising a diversity receiver to provide the signals, diode gatesreceiving said signals and connected to output terminals, a flip-flopswitch connected to said gates to make either gate conductive and theother non-conductive, limiters receiving portions of the signals fromthe diversity receiver, selective detectors receiving the signalsdelivered by the limiters, said selective detectors being tuned to amaximum energy portion of the complete audio spectrum, the outputs ofthe detectors being connected to the flip-flop switch to control thegates, and a phase reversing switch between the flip-flop switch and thediode gates, whereby the latter may select the signal with more noise orthe signal with less noise.

5. Apparatus for selecting from two voice signals the signal having thegreater or lesser signal-to-noise ratio, said apparatus comprising adiversity receiver to provide the signals, diode gates receiving saidsignals and connected to output terminals, a flip-flop switch connectedto said gates to make either gate conductive and the othernon-conductive, limiters receiving portions of the signals from thediversity receiver, selective detectors receiving the signals deliveredby the limiters, said selective detectors being tuned to amaximum-energy portion of the complete audio spectrum within say 300 to600 cycles per second,

the outputs of the detectors being connected to the flip flop switch tocontrol the gates.

6. Apparatus for selecting from two voice signals the. signal having thegreater or lesser signal-to-noise ratio, said apparatus comprising adiversity receiver to provide the signals, diode gates receiving saidsignals and connected to output terminals, a flip-flop switch connectedto said gates to make either gate conductive and the othernon-conductive, limiters receiving portions of the signals from thediversity receiver, selective detectors receiving the signals deliveredby the limiters, said selective detectors being tuned to amaximum-energy portion of the complete audio spectrum within say 300 to600 cycles per second, the outputs of the detectors being connected tothe flip-flop switch to control the gates, andv a double-pole,double-throw reversing switch between the flip-flop switch and the diodegates, whereby the latter may select the signal with more noise or thesignal with less noise.

7. Apparatus for selecting the stronger or weaker of two audio frequencysignals, said apparatus comprising a diversity receiver to providethe'signals, selective detectors receiving the signals from thediversity receiver, said selective detectors being tuned to a narrowmaximum energy portion of the complete audio spectrum, said portionhaving a width of only a few hundred cycles per second, and meansconnected to said selective detectors for selecting the desired signalin response to a control potential obtained from the selectivedetectors.

8. Apparatus for selecting the stronger or Weaker of two voice signals,said apparatus comprising a diversity receiver to provide the signals,selective detectors receiving the signals from the diversity receiver,said selective detectors being tuned to a maximum energy portion of thecomplete audio spectrum within, say, 300 to 600 cycles per second, meansconnected to said selective detectors for selecting the desired signalin response to a control potential obtained from the selectivedetectors, and a reversing switch included in said means whereby thelatter may select either the stronger or weaker signal.

9. Apparatus for selecting the stronger or weaker of two audio frequencysignals, said apparatus comp-rising a diversity receiver to provide thesignals, idiode gates receiving said signals and connected to outputterminals, a flip-flop switch connected to the gates to make either gateconductive and the other non-conductive, selective detectors receiving aportion of each of the signals from the diversity receiver, saidselective detectors being tuned to a maximum energy portion of thecomplete audio spectrum, said portion having a width of only a fewhundred cycles per second, the output of the detectors being connectedto the flip-flop switch to control the gates.

10. Apparatus for selecting the stronger or weaker of two audiofrequency signals, said apparatus comprising a diversity receiver toprovide the signals, diode gates receiving said signals and connected tooutput terminals, a flip-flop switch connected to the gates to makeeither gate conductive and the other non-conductive, selective detectorsreceiving a portion of each of the signals from the diversity receiver,said selective detectors being tuned to a maximum energy portion of thecomplete audio spectrum, said portion having a width of only a fewhundred cycles per second, the output of the detectors being connectedto the flip-flop switch to control the gates, and a. reversing switchbetween the flip-flop switch and the gates, whereby the latter mayselect either the stronger or the weaker signal.

11. Apparatus for selecting the stronger or weaker of two voice signals,said apparatus comprising a diversity receiver to provide the signals,diode gates receiving said signals and connected to output terminals, aflip-flop switch connected to the gates to make either gate conductiveand the other non-conductive, selective detectors receiving a portion ofeach of the signals from the diversity receiver, said selectivedetectors being tuned to a maximum-energy portion of the audio spectrumwithin say 300 to 600 cycles per second, the outputs of the detectorsbeing connected to the flip-flop switch to control the gates.

12. Apparatus for selecting the stronger or weaker of two voice signals,said apparatus comprising a diversity receiver to provide the signals,diode gates receiving said signals and connected to output terminals, aflip-flop switch connected to the gates to make either gate conductiveand the other non-conductive, selective detectors receiving a portion ofeach of the signals from the diversity receiver, said selectivedetectors being tuned to a maximum-energy portion of the audio spectrumwithin say 300 to 600 cycles per second, the outputs of the detectorsbeing connected to the flip-flop switch to control the gates, and adouble-pole, double-throw reversing switch between the flip-flop switchand the gates, whereby the latter may select either the stronger or theweaker signal.

13. Apparatus for selecting from two audio frequency signals, the signalhaving either the greater or lesser signal-to-noise ratio or thestronger or weaker signal, said apparatus comprising a diversityreceiver to provide the signals, limiters receiving the signals from thediversity receiver, selective detectors receiving the signals deliveredby the limiters, said selective detectors being tuned to a maximumenergy portion of the complete audio spectrum, means connected to saidselective detectors for selecting the desired signal in response to acontrol potential obtained from the selective detectors, and meansassociated with each of the limiters for disabling the same whereby theselection is changed from one based on signal-to-noise ratio to onebased on signal strength.

14. Apparatus for selecting from two voice signals, the signal havingeither the greater or lesser signal-tonoise ratio or the stronger orweaker signal, said apparatus comprising a diversity receiver to providethe signals, limiters receiving the signals from the diversity receiver,selective detectors receiving the signals delivered by the limiters,said selective detectors being tuned to a maximum energy portion of thecomplete audio spectrum within, say, 300 to 600 cycles per second, afirst means connected to said selective detectors for selecting thedesired signal in response to a control potential obtained from theselective detectors, a second means associated with each of the limitersfor disabling the same whereby the selection is changed from one basedon signal-to-noise ratio to one based on signal strength, and areversing switch included in said first means whereby the latter mayselect the signal with more or less of either noise or strength.

15. Apparatus for selecting from two audio signals the signal havingeither the greater or lesser signal-tonoise ratio, or the stronger orweaker signal, said apparatus comprising a diversity receiver to providethe signals, diode gates receiving said signals and connected to outputterminals, a flip-flop switch connected to said gates to make eithergate conductive and the other nonconductive, limiters receiving portionsof the signals from the diversity receiver, selective detectorsreceiving the signals delivered by the limiters, said selectivedetectors being tuned to a maximum-energy portion of the complete audiospectrum, the outputs of the detectors being connected to the flip-flopswitch to control the gates, and means associated with each of thelimiters for disabling the same whereby the selection is changed fromone based on signal-to-noise ratio to one based on signal strength.

16. Apparatus for selecting from two audio signals the signal havingeither the greater or lesser signal-tonoise ratio, or the stronger orweaker signal, said apparatus comprising a diversity receiver to providethe signals, diode gates receiving said signals and connected to outputterminals, a flip-flop switch connected to said gates to make eithergate conductive and the other non-conductive, limiters receivingportions of the signals from the diversity receiver, selective detectorsreceiving the signals delivered by the limiters, said selectivedetectors being tuned to a maximum-energy portion of the complete audiospectrum, the outputs of the detectors being connected to the flip-flopswitch to control the gates, means associated with each of the limitersfor disabling the same whereby the selection is changed from one basedon signal-to-noise ratio to one based on signal strength, and areversing switch between the flip-flop switch and the diode gates,whereby the latter may select the signal with more or less of eithernoise or strength.

17. Apparatus for selecting from two voice signals the signal havingeither the greater or lesser signal-to-noise ratio, or the stronger orweaker signal, said apparatus comprising a diversity receiver to providethe signals, diode gates receiving said signals and connected to outputterminals, a flip-flop switch connected to said gates to make eithergate'conductive and the other non-conductive, limiters receivingportions of the signals from the diversity receiver, selective detectorsreceiving the signals delivered by the limiters, said selectivedetectors being tuned to a maximum-energy portion of the complete audiospectrum within say 300 to 600 cycles per second, the outputs of thedetectors being connected to the flip-flop switch to control the gates,and means associated with each of the limiters for disabling the samewhereby the selection is changed from one based on signal-to-noise ratioto one based on signal strength.

18. Apparatus for selecting from two voice signals the signal havingeither the greater or lesser signal-to-noise ratio, or-the stronger orweaker signal, said apparatus comprfsing a diversity receiver to providethe signals, diode gates receiving said signals and connected to outputterminals, a flip-fiop switch connected to said gates to make eithergate conductive and the other non-conductive, limiters receivingportions of the signals from the diversity receiver, selective detectorsreceiving the signals delivered by the limiters, said selectivedetectors being tuned to a maximum-energy portion of the complete audiospectrum within say 300 to 600 cycles per second, the outputs of thedetectors being connected to the flipflop switch to control the gates,means associated with each of the limiters for disabling the samewhereby the selection is changed from one based on signal-to-noise ratioto one based on signal strength, and a double-pole, double-throwreversing switch between the flip-flop switch and the diode gates,whereby the latter may select the signal with more or less of eithernoise or strength.

References Cited in the file of this patent UNITED STATES PATENTS1,747,236 Gillett Feb. 18, 1930 2,306,687 Cox Dec. 29, 1942 2,494,309Peterson et al. Jan. 10, 1950 2,515,055 Peterson July 11, 1950 2,610,202Bond et al. Sept. 9, 1952

